Isaac Newton devised a diagram showing the visible spectrum as a circle in his work Opticks published in 1704. Newton's color circle 10 is illustrated in FIG. 1. Although Newton did not make note of the discontinuity between the colors red and violet, the diagram 10 is a useful tool for illustrating the manner in which colors mix. For example, color points can be synthesized by drawing lines between available colors on the diagram 10 and altering their proportions.
The ability of the human eye to sense colors, and the ability of the human brain ability to perceive colors, are dependent upon the wavelength of the light. The human eye is sensitive to light in the spectrum of wavelengths from approximately 390 nm (i.e., violet) to approximately 700 nm (i.e., red), as illustrated in diagram 15 of FIG. 2. Color perception by the human brain with respect to wavelength is illustrated in diagram 20 FIG. 3, although color perception and the ability to detect the extreme ends of the visual spectrum vary from individual to individual. For example, for low levels of light, the visual spectrum of wavelengths can extend further into the ultra violet (“UV”) range as rod detectors in the human eye, which have a more significant response to UV light, dominate color perception.
With the exception of partial UV and very low levels of black-and-white vision produced by the eye's rod detectors, primary color perception is produced by three types of cone detectors which detect broad bands of color in the red, green, and blue wavelengths (see FIG. 3). The cone detectors produce signals (e.g. pulse signals) proportional to the number of photons arriving on the each of the cone detectors having wavelengths within their sensitivity range.
The human brain interprets the rate at which the cone detectors produce pulse signals to create the perception of a color. The human brain is also capable of perceiving colors with wavelengths of light outside of the simple color spectrum. For example, colors such as lavender, pink, and magenta are not spectral colors, and can only be made by the mixing different spectral colors (e.g., red and blue). Wavelengths of light which fall in between the peak responses of the cone detectors, such as yellow-green, cyan, and magenta are perceived according to the relative proportion of signals from the red and green pair of cone detectors, the green and blue pair of cone detectors, and the blue and red pair of cone detectors, respectively.
Such a process allows the human brain to perceive a large number of apparent colors being output from a luminaire with a small number of light sources (e.g., three light sources). In a similar manner, it is possible to fill in the color gaps of a practical luminaire by spectrally positioning the light sources on either side of these color gaps. For example, a yellow color gap can be filled by mixing red, amber, and green in suitable proportions.
The light sources in luminaires typically use devices or emitters which produce narrow-band electrochemical emissions, such as light-emitting diodes (“LEDs”), organic light-emitting diodes (“OLEDs”), fluorescent sources, or other similar devices. Such light sources are generally only available in a limited variety of colors, and between these colors, there are parts of the visible spectrum that have no emitters available. For example, using current technology, yellow and yellow/green (i.e., wavelengths from 550-580 nm) are difficult to produce. As a result of gaps which appear in the visual spectrum where there is no substantial light emission, a practical color mixing luminaire offers control over some but not all parts of the visible spectrum, because. Additionally, the emitters at the limits of the visible spectrum typically have a lower lumen output (e.g., perceived power), are less efficient, and are more expensive to produce.
Practical emitters also suffer from variations in spectral bandwidth, absolute luminosity, and dominant wavelength. As such, manufacturers batch-sort or bin emitters into moderately wide ranges. Although batch sorting into narrow, precise ranges is technically feasible, it is unreasonably expensive. For example, current LED technology provides up to approximately nine colors having the characteristics shown below in Table 1, although violet and extreme red are uncommon, expensive, and perform relatively poorly in comparison to the other colors.
TABLE 1LED LIGHT SOURCESWAVELENGTHHALF-WIDTHBINNING RANGEHUE(nm)(nm)(nm)Violet41025390-420Royal Blue45020440-460Blue47025460-490Cyan50530490-520Green53035520-550Amber59014585-600Red/Amber61520610-620Red63020620-645Extreme Red  660+20No data
Luminaires which incorporate multiple light sources are also typically controlled using one or more of three basic techniques. The first technique provides simple controls for the individual color sources such that a user is able to alter the intensity of each component color from zero to full-scale using a separate control. Typically, a linear or rotary fader or dial is used for this control. Alternatively, a numerical intensity value for each individual color is entered by the user. Such a technique is cumbersome and difficult because a user must have at least a working knowledge of color theory to obtain a desired final color when independently manipulating several sources.
The second technique provides control of the hue, saturation, and intensity (“HSI”) using three of the above described controls or using a graphical map of the visual color space. This technique allows the user to pick a color represented by the color space between three points (i.e. within the triangle formed by the points of the primary colors red, green, and blue, or alternatively, the secondary colors cyan, magenta and yellow), and vary the saturation and intensity of that color.
The third technique provides commonly named or numbered colors, which correspond to lighting filters (e.g., gels) used in theatre or television lighting. The user selects a name or number of a color, and the color is identified in a table which includes the component color values necessary to produce the selected color.
Each of the above techniques is based on the use of three base colors from which all other colors are subsequently generated. Such techniques are commonly used in cathode ray tube (“CRT”) displays, flat panel displays, and variable color luminaires which use either primary emissive sources (e.g., LEDs) or secondary filtered sources (e.g., gels).